The invention relates to the field of materials research or analysis using electrical means and has as a particular subject a microfluidic device for separating, fractionating, or preconcentrating analytes contained in an electrolyte.
Surface potential is involved in many analytical separation processes. For example, microfluidic devices for separating analytes contained in an electrolyte employing capillary electrophoresis are known. This process is generally implemented by a microfluidic network into which an electrolyte and a specimen containing analytes are injected. This network may have a number of reservoirs connected to at least one long microchannel and/or to a microchannel network having intersections in a particular arrangement to enable injection of a certain quantity of analytes into the central microchannel. Application of an electric field in this same channel, known as separation channel, after the injection phase, is responsible for migration of the analytes. Under the effect of an electric field, the charged particles move in a liquid medium at a speed defined both by the field and by the mass and charge of the particles (electrophoresis). The particle speed in the liquid is proportional to the electric field, the proportionality constant being called electrophoretic mobility. At the solid-liquid wall, a double ionic layer formed of a fixed layer of ions, corresponding to the surface charge, and of a mobile ion layer, corresponding to a diffuse layer in the liquid, forms spontaneously. Under an electric field, the ions in the mobile layer migrate, bringing about a general liquid movement by viscosity (electroosmosis). The latter moves in a single block and its speed is also proportional to the electric field. The proportionality constant between the fluid speed and the electric field is called electroosmotic mobility. The concomitant action of the electrophoretic migration (speed of ions in the liquid) and the electroosmotic liquid flowrate (liquid speed) that are generated by the difference in potential acts on the ions contained in the fluid, ensuring that they are carried through the separation channel. The total speed of an ion in a microchannel subjected to an electric field is hence proportional to the electric field. The proportionality constant is the total mobility of the ion which is the sum of its electrophoretic mobility (specific to each ion) and the electroosmotic mobility (promotional to the surface potential).
The various analytes can be detected sequentially in time at the end of the long microchannel, giving information on the number of analytes present in the solution to be analyzed and their respective concentrations. This method is known for its very good resolution in separating two analytes. To achieve the highest possible separation efficiency, one must control the direction and amplitude of the electroosmotic flow (EOF). The resolution of this technique is maximal when the surface potential is such that the electroosmotic mobility is the converse of the average electrophoretic mobility of the species to be separated (see Huang, L. R., The Principles of Separation in CE, Chromatographia Supplement, 54 (9), pp. 15-23).
Numerous preconcentration systems also require the surface potential parameter to be controlled. French Patent Application No. FR0805264 consists of a selective preconcentration technique based on the variation in the surface charge parameter (NB: the surface charge can be expressed as surface potential and vice versa).
In nanofluidics, it is also known that the selectivity of ion transport depends on the surface potential and the preconcentration of a biological specimen with the aid of such nanometric structures depends essentially on this parameter (Plecis A., Nanteuil C., Haghiri-Gosnet A., Chen Y., “Electro-preconcentration with Charge Selective Nanochannels,” Analytical Chemistry, 2008). For all these reasons, numerous techniques have been proposed and numerous patents have been filed to control this parameter based on an external control system of the voltage source type. These techniques are known as “radial electric field” when they are applied to systems of the capillary type, or as microfluidic or nanofluidic transistors when applied to microfluidic systems (for example by Schasfoort, R. B. M., et al., Field-Effect Flow Control for Microfabricated Fluidic Networks, Science, 1999 or by Karnik, R, K. Castelino, and A. Majumdar, Field-Effect Control of Protein Transport in a Nanofluidic Transistor Circuit, Applied Physics Letters, 2006). We will now describe in detail two examples of patents inspired by these structures of the Metal/Insulator/Electrolyte (MIE) type in order to control the surface potential (also generally called “zeta” potential).
Thus, FIG. 1, from U.S. Pat. No. 5,151,164, describes a device for separation by capillary electrophoresis of analytes contained in an electrolyte, said device having two reservoirs 1, 2 separated by a capillary 3. This capillary 3 is made of a non-conducting material, for example glass, plastic, or silica. A first and a second electrode 4, 5, with which a voltage generator 6 is associated, are disposed on one and the other side respectively of the microchannel 10 and are able to generate, inside the latter, a first longitudinal electric field. A third electrode 7, disposed around capillary 3 and over almost its entire length, is able to generate a radial electric field inside the latter.
Controlling the direction and amplitude of the electroosmotic flow is achieved by controlling the voltage applied to the ends of the third electrode 7. A drop in electric potential of at least 0.4 kV is applied through this electrode, enabling a constant potential difference with the liquid in capillary 3 to be obtained and also the inside surface potential of the capillary to be controlled.
However, this sharp voltage drop in the external electrode causes a substantial Joule effect and, to avoid this drawback, U.S. Pat. No. 5,358,618 proposes using a single potential electrode positioned over a restricted length of the microchannel. This device controls electroosmotic flow while limiting heating of the liquid due to the Joule effect.
Referring to other microsystems in which the surface potential is controlled with the aid of a radial potential, all have this same configuration, known as “Metal-Insulator-Electrolyte” (MIE), which consists of insulating an electrode from the electrolyte with an insulating layer and applying a potential to this electrode that is different from the potential of the electrolyte in order to modify, by a capacitive effect, the surface potential of the insulator. FIG. 2a shows the evolution of the electric potential through an MIE structure surrounding the microchannel. FIG. 2b shows an electric modeling of this type of interface. Vg corresponds to the potential applied to the metal, Ψ to the Stern potential at the limit of the insulator, ζ to the zeta potential (or surface potential beyond the layer of ions adsorbed at the surface), and Φ to the liquid potential (beyond the contra-ion layer, called diffuse layer). The decrease in potential is linear in the insulator and the Stern layer, while the decrease is exponential in the diffuse layer.
If the capacitance of the Stern layer, whose value is not known at the outset, is ignored, the variation in surface potential ζ implied by an overvoltage between the liquid and the electrode (V−Φ) can be estimated:
  Δζ  =                              C          insulator                          C          DL                    ⁢              (                  V          -          ϕ                )              =                                        λ            D                    ⁢                      ɛ            insulator                                                l            insulator                    ⁢                      ɛ            liquid                              ⁢                        (                      V            -            ϕ                    )                .            where λD is the thickness of the diffuse layer (Debye length—see Hunter, “Zeta Potential in Colloids Science”, Academic Press, London, 1981), linsulator is the thickness of the insulator, ∈insulator and ∈liquid designate respectively the electric permittivity of the insulator and of the electrolyte. The variation in potential at the liquid interface is hence proportional to the difference in potential between the liquid and the electrode. There are three main difficulties with these interfaces:                the first has to do with the fact that the potential in the liquid varies linearly along the microchannel. Hence, the induced surface potential is not homogeneous along the capillary. This phenomenon is illustrated in FIG. 3 where a channel 20 has a first electrode 21 at one of its ends and a second electrode 22 at its other end, these electrodes being connected to means 25 for generating and controlling a voltage between them. A non-conducting material 23 is provided on part of the outer face of the channel and a third electrode 24 is connected thereto. Means 26 for generating and controlling a voltage are associated with this third electrode. The inhomogeneity of the surface potential is represented by the signs − and + below the non-conducting material 23.        The second limitation derives from the proportionality factor between the applied external voltage and the surface potential difference generated. This factor is generally very small and even tends to zero when the ionic concentration increases (it is proportional to the Debye length). Thus, for a 10 mM solution, and an insulating layer of the SiO2 type (2 μm thickness), a voltage difference of about 100 V has to be applied to obtain a difference of only 10 mV at the interface. Since the dielectric strength of glass does not enable electric fields greater than 8 MV/m on average to be applied for a layer 2 μm thick, the maximum permissible voltage before breakdown would a priori be 16 V, hence a maximum surface potential control of only 1.6 mV.        Finally, the third difficulty is linked to the technological near-impossibility of making a thin and truly insulating layer in a liquid medium with current microfabrication techniques. Indeed, the slightest imperfection in this layer results in a large faradaic current between the metal electrode and the liquid, which disturbs the value of the electrolyte's electric potential. This obviously must not occur in electrophoretic separation, fractionation, or preconcentration steps.        
Moreover, a device according to U.S. Pat. No. 5,151,164 and U.S. Pat. No. 5,358,618 and any classical capillary electrophoresis device also has the drawback of becoming non-functional when a bubble, created for example by the Joule effect, is created inside the microchannel. If this happens, the bubble acts as an electrical insulator and no electric current can be established in the capillary so that any electroosmotic flow is “stopped”.
To overcome this drawback, Patent application WO 00/60341 proposes using a cylindrical microchannel whose entire inside face is covered with a resistive coating such as a semiconductor polymer (polyaniline, polypyrrole, etc.), and connecting the ends of this coating to a voltage source that also allows the electric field to be imposed in the liquid part. In this case, the resistive interface conducts the current despite the presence of a bubble and the latter can be carried by the electroosmotic flow. This type of device, while it solves the problem of bubbles in capillary devices, nonetheless does not enable the surface mobility to be properly controlled.
Classical microfabrication techniques cannot be applied to this type of system and covering the entire inside face of a cylindrical microchannel homogenously is extremely difficult or even impossible to accomplish with solid materials. So, coating techniques based on hydrodynamic injection of polymer solutions are used, where the solutions are deposited passively on the inside of the capillary. This type of technique does not provide sufficient homogeneity for the surface conductivity and does not allow integration of highly conductive solid materials. Moreover, it is impossible to connect the resistive layer at points other than the ends of the capillary. Hence, the control of the electric potential in this resistive layer is strongly limited.
Furthermore, the use of the above-mentioned polymers requires strong faradaic currents between the liquid medium and the resistive layer. This electron transfer between the liquid and the resistive layer is essential in the context of the application described which consists of using the resistive layer as a “short circuit” when a bubble forms in the liquid part of the capillary. This property implies minimal polarizability at the solid-liquid interface. Moreover, the low conductivities referred to for the resistive layer mean that the potential difference between the liquid and the resistive layer cannot be maintained along the microchannel.